Electronic image sensors are incorporated not only in digital cameras but, more recently, also into such consumer electronic products as mobile telephones and personal digital assistants (PDAs). Such products are subject to market pressure to reduce them in size, or to pack more features into a product of a given size.
FIG. 1 is a schematic isometric view of a highly simplified example of a typical conventional color image sensor 10 found in present day products such as digital cameras, mobile telephones and PDAs. Image sensor 10 is composed of a single light sensor 12 and a single imaging element 14 that forms an image of the subject on the major surface 16 of the light sensor.
Light sensor 12 is typically composed of a two-dimensional (typically rectangular) array of sensor elements and an associated read-out circuit (not shown). The boundary of an exemplary sensor element 20 is shown by a broken line. The example of light sensor 12 shown is highly simplified in that it has only 48 sensor elements. A typical light sensor has hundreds of thousands or millions of sensor elements. Each sensor element is typically a complementary metal-oxide-semiconductor (CMOS) sensor element or a charge-coupled device (CCD) sensor element. The read-out circuit receives the electrical values generated by the sensor elements in response to light from the subject and converts the electrical values into an analog or digital image signal, typically in raster-scan order. To enable the light sensor to generate a color image signal that additionally provides color information regarding the subject, the light sensor additionally incorporates a color mosaic filter 30 on major surface 16.
The most common type of color mosaic filter is the Bayer color mosaic filter, named after its inventor. The example of the Bayer color mosaic filter 30 shown in FIG. 1 has N/4 red filters, N/2 green filters and N/4 blue filters, where N is the number of sensor elements 20 in light sensor 12. The red filters, green filters and blue filters will be generically referred to herein as color filters. Each color filter filters the light incident on a respective one of the sensor elements of the light sensor. The color filters are arranged in square blocks of four, an exemplary one of which is shown at 32. Block 32 is composed of a red filter 34, two green filters 36 and 37 diagonally opposite one another, and a blue filter 38. The remaining blocks of color filters are similar in structure. The colors of the filters are indicated in FIG. 1 by different hatching. Many variations of the Bayer pattern have been tried, including a pseudo random arrangement of the color filters and use of filters of four or more colors.
Each sensor element 20 generates an electrical value that represents the intensity of the light incident on the sensor element. The light incident on the sensor element is of the color transmitted by the color filter overlying the sensor element. For example, the sensor element under the red filter 34 generates an electrical value that represents the intensity of red light incident on the sensor element. Since a color image signal representing a color image typically includes electrical values representing the intensities of red light, green light and blue light incident on the color filter over each sensor element of light sensor 12, the light sensor additionally includes a processing circuit (not shown) that receives the electrical values from the read-out circuit (not shown) connected to the sensor elements and synthesizes the missing electrical values for each sensor element. The processing circuit synthesizes the missing electrical values by interpolation using the electrical values generated by neighboring ones of the sensor elements.
The processing circuit generates the color image signal by synthesizing a red value and a blue value for each sensor element covered by a green filter, a green value and a blue value for each sensor element covered by a red filter and a red value and a green value for each sensor element covered by a blue filter. For example, the processing circuit synthesizes a green value for the sensor element covered by red filter 34 from the electrical values generated by the sensor elements covered by green filters 36 and 37, and possibly additionally from other neighboring sensor elements covered by green filters. The electrical values from which the green value is synthesized are all green values. Two thirds of the resulting color image signal is synthesized by interpolating from the electrical values generated by the neighboring sensor elements covered with filters of the appropriate color.
The need to synthesize the missing electrical values in conventional light sensor 12 causes many problems with certain types of subject when a picture is displayed in response to the color image signal. Adjacent areas of the picture with sharp color differences can produce false colors. Almost horizontal or vertical lines are displayed with jagged edges and can exhibit color problems. Conventional color image sensor 10 typically additionally includes a spatial filter (not shown) interposed between imaging element 14 and light sensor 12. The spatial filter reduces the high spatial frequency content of the image formed on light sensor 12. This ameliorates some of the effects just described at the expense of a displayed picture that is less sharp than the sharpness implied by the number of sensor elements 20 in light sensor 12 and the optical quality of imaging element 14.
Another problem of conventional light sensor 12 is that light leakage from one sensor element to a neighboring sensor element not only causes blurring in the displayed picture but additionally causes false color.
A further problem with conventional color image sensor 10 arises because imaging element 14 forms an image on light sensor 12 in polychromatic light. Consequently, imaging element 14 must be color corrected to ensure that the image it forms on light sensor 12 is sharp for all colors. This typically requires that imaging element 14 be a multi-component lens, which is typically larger than a single-component lens in the direction of light transmission through the lens. This increases the overall depth of the color image sensor. A large image sensor depth is particularly undesirable in the highly-miniaturized applications described above. The need for color correction additionally limits the choice of lens material.
A further problem with conventional color image sensor 10 is that all the sensor elements use the same exposure time regardless of the color of the overlying color filter. This limits the dynamic range of the image sensor. For example, if a subject includes a bright green region, the exposure time of light sensor 12 must be reduced to avoid saturating the sensor elements having green filters. The same reduced exposure is also applied to the sensor elements having red and blue filters, so that the electrical values generated by those of the sensor elements on which a subtle red portion of the subject is imaged will include an unwanted noise component.
A final problem with conventional color image sensor 10 is its low light performance. The color filters cause the underlying sensor elements to receive typically only a fraction of the light incident on the color filters.
Thus, what is needed is a color image sensor that does not suffer from the problems described above.